1. Field of the Invention
This invention relates to low permeability subterranean formation testing methods and apparatus, and more particularly, to methods and apparatus for testing core samples of low permeability formations to determine the gas permeabilities thereof and the gas permeability reducing effects of injecting one or more treatment fluids therein.
2. Description of the Prior Art
Various methods and apparatus have heretofore been developed for testing core samples of subterranean formations to determine various properties thereof including the permeabilities of the formations and the permeability reducing effects of injecting treatment fluids into the formations. Such methods and apparatus have included the use of a core sample test container known as a "Hassler sleeve" in which a core sample is confined and pressure is applied to the periphery thereof to simulate the confining stress of the formation from which the core sample was taken. The permeability of the core sample is determined, a treatment fluid is injected into the core sample and after the removal of treatment fluid, the permeability of the core sample is again determined. A comparison of the core sample permeabilities before and after the injection of the treatment fluid is then made to determine the reduction, if any, in the permeability of the core sample caused by the treatment fluid.
U.S. Pat. No. 4,842,073 issued on Jun. 27, 1989 to Himes et al. discloses such a core sample testing method and apparatus. As shown in the drawing, the apparatus of Himes et al. includes a Hassler sleeve and means for injecting nitrogen through a core sample confined within the Hassler sleeve to simulate the production of gas from the formation. Means for injecting a treatment fluid through the core sample in the opposite direction are also provided to simulate the injection of a treatment fluid into the formation. The percent of permeability recovery of the core sample after injecting a treatment fluid therein is determined by comparing the nitrogen permeability of the core sample before and after the treatment fluid injection.
While the prior methods and core sample testing apparatus have been utilized successfully in testing core samples of subterranean formations having normal permeabilities, such methods and apparatus have been unsuccessful in testing subterranean formation core samples of extremely low gas permeabilities, e.g., gas permeabilities in the range of from about 1.times.10.sup.3 to about 1.times.10.sup.-3 micro-darcies. Thus, there is a need for improved test methods and apparatus which can be utilized for determining the gas permeabilities of such subterranean formations as well as the effects of treatment fluids thereon.